think twice about getting Sigma 18-200mm ... it is not a very good lens at all.
tamron 18-200 is a bit better but still not very good.
Sigma 18-125 is much better choice if you are concerned about image quality.
Sigma 17-70mm is a great lens. Canon 18-85mm IS is another great lens.
MUCH better image quality, distortion, sharpnes focus speed.

I tried the sigma and tamron 18-200 - the focus speed is horrible.

It may be alright for you though as you may be shooting the subjects that never move and have all the time in the world to focus and then you don't really care much about the end result.